There are a number of patents for devices for propelling an inflated ball. These devices comprise an element for holding the inflated ball in a fixed position, a guideway with variable horizontal and vertical orientation, and mobile elements for propelling the ball. In these devices the propulsion is accomplished, for example, using springs, compressed air, rollers or pulleys rotating at high speed, or oscillating arms comprising a single piece driven by a spring or a rotating motor.
All these devices have disadvantages which greatly limit their use: the compressed air devices are very noisy; the spring devices have a low frequency of firing and a power which changes rapidly due to weakening of the spring; and the devices equipped with electric motors are weak, and they are very sophisticated, which makes them costly and difficult to maintain.
An oscillation arm which is long so as to attain the speed needed to propel a ball is fragile if it is of a light construction; and if it is of a stronger construction its mass is difficult to retard after striking the ball.